In many cases with crystallizable liquids, e.g. aqueous solutions, it is desirable to thicken the solution and separate out therefrom, at low temperatures, crystals, e.g. of ice, and produce an inspissate or concentrate. Depending upon the application, the product of interest may be the inspissate or the crystals. For example, in the desalination of water, ice crystals may be recovered and melted to produce salt-free water starting from brine. In chemical processes, the concentrated solutions may be desirable as a mother liquor for salt formation or the like.
One technique for producing a concentrate from a relatively dilute solution with recovery of crystals, is to subject the solution to low temperature, i.e. a temperature below the freezing point of the solvent, by contact with cooled heat exchange surfaces, to cause the crystals to form. To this end, the solution is passed through a heat exchanger which, in order to prevent accumulation of the deposits upon these surfaces and thereby interfere with the heat exchange, can be vibrated or can have surfaces subjected to vibration. As a result, the crystal deposits can be dislodged from the cooling surfaces and can be recovered.
The most common practice in this field, especially in the case of the treatment of aqueous solutions by the freezing out of ice crystals, is to pass the solution upwardly through a heat exchanger having a plurality of vertical tubes (i.e. a tube-bundle heat exchanger). The solution is introduced at the lower end of this heat exchanger and flows threough the tubes in counterflow to a descending stream of a coolant which traverses the spaces between the tubes. The upwardly flowing liquid carries with it any ice which is formed upon the cooled surfaces of the tubes and both the ice and the concentrated liquid are recovered at the upper end of the tube bundle.
Since a layer of ice has a very low coefficient of conductive heat transfer, the formulation of ice layers upon the walls of the tubes which provide the indirect heat exchange between the coolant and the liquid poses a problem. To prevent the formation of the ice layers or to break these layers loose from the walls of the tubes as the ice is formed, it is a common practice to impart vertical oscillation to the tubes. These oscillations or vibrations effectively displace the surface of the pipe or tube relative to the liquid or crystals at the interface, relying upon the inertia of the crystals or liquid which cannot follow readily the movements of the heat exchange surfaces. Problems are frequently encountered when the tubes of a tube-bundle heat exchange are vibrated and shear forces are established because of the inertia of the liquid and crystals and the vibratile movement of the pipe walls. Furthermore, efficient freezing operations with systems of this type have required numerous freezing stages in cascade with intervening removal of the ice to prevent problems with entrainment of large volumes of ice through the pipes by the liquid to be concentrated.